Parts of my backyard and woodland are covered with fallen pine needles. We have a large number of mature white pine trees that yearly drop their needles. Annual needle drop is normal and beneficial.
Martha Smith, Horticulture Educator with University of Illinois Extension said it well, "There is really nothing to be concerned about," "What is happening is commonly called inner needle drop or third year needle drop."
All trees and shrubs renew their foliage annually, producing new leaves in the spring of the year and shedding old leaves in the fall. The leaves of deciduous plants such as maples and oaks live for one growing season and then fall off usually in a blaze of color.
Despite the name, evergreen foliage does not live forever. Actually evergreen foliage lives from one to several years, depending on the species. As new growth emerges in the spring, last years growth becomes shaded and is no longer the plant's primary food. During late September and October, this inner or older foliage dies and falls away.
In some species like white pine and arborvitae, this fall browning takes place rather suddenly. The older needles turn a bright gold-yellow and remain attached for about 7 to 10 days depending on the weather. If we have strong autumn winds and heavy rains, these needles fall quickly. Sometimes, this natural occurrence is hardly noticed. But every few years it is very noticeable, and people become concerned.This natural foliage drop may be distinguished from cases of severe foliage damage due to disease by its uniform appearance over the whole tree and its common occurrence on neighboring trees of the same kind. It is also confined to the innermost or oldest needles. Nearly all pines bear needles in bundles of two to five, and the needles remain together when they drop.
This dropped foliage is beneficial, acting as natural mulch. In fact, pine needle mulch is commonly used in the South where it is overly abundant. The dry needles decompose rapidly and add useful organic matter to the soil. As with other mulches, they help stabilize soil moisture and temperature and aid in weed control.
So, don't be alarmed if your evergreens are dropping foliage this fall. It is normal. If you can't leave them lie, put them to good use as mulch or in the compost pile.
Source: Martha A. Smith, Horticulture Educator, University of Illinois Extension